Consumer and industrial applications continue to drive demand for new and efficient batteries for use as energy sources. Important goals include obtaining ever more power from increasingly smaller battery packages in an environmentally respectful fashion. Envisioned applications for batteries include everything from mobile electronics to electric vehicles. Portability, rechargeability over a large number of cycles, low cost, high power, lightweight and consistent performance over widely varying loads are among the key attributes required for batteries. The specific combination of battery performance requirements varies widely with the intended application and the battery components and materials are typically optimized accordingly.
An important developing application area for rechargeable batteries is electric vehicles (EV) and hybrid electric vehicles (HEV). In these applications, the battery must have the ability to provide high currents in short time periods in order to achieve effective acceleration. High discharge rates are therefore necessary. High battery power over extended time periods are also needed so that vehicles of reasonable size and weight can be maintained in motion for reasonable time intervals without recharging. Rapid recharging over many cycles should also be possible using readily available electrical power sources. The preferred cycle life profile also requires a high number of charge/discharge cycles at a shallow depth of charge/discharge. Progress has been made in the development of batteries for HEV applications and a few HEV automobiles have recently been made available to the U.S. public. Nonetheless, the batteries used in these automobiles represent compromises and trade-offs in relevant performance parameters and new developments are needed to further the capabilities of HEV and EV products.
Nickel metal hydride batteries have emerged as the leading class of rechargeable batteries and are replacing earlier generation nickel-cadmium batteries in many applications. Relative to nickel cadmium batteries, nickel metal hydride batteries avoid significant environmental problems (due to the toxicity of cadmium) while providing higher energy densities. HEV and EV products are examples of applications that utilize the high energy density available from nickel metal hydride batteries and are also applications viewed to be impractical in a nickel-cadmium paradigm due to the disposal problems associated with cadmium. Expanded performance of HEV and EV products and the future extension of rechargeable batteries to new applications in the future will greatly depend on improvements in the capabilities of nickel metal hydride batteries.
Nickel metal hydride batteries typically include a nickel hydroxide positive electrode, a negative electrode that incorporates a metal-containing hydrogen storage alloy, a separator made from nylon, polypropylene or other polymers, and an aqueous alkaline electrolyte. The positive and negative electrodes are housed in adjoining battery compartments that are typically separated by a non-woven, felled, nylon or polypropylene separator. Several batteries may also be combined in parallel or series to form larger battery packs capable of providing higher powers, voltages or discharge rates.
The charging and discharging reactions of nickel metal hydride batteries have been discussed in the art and may be summarized as shown below:                Charging:                    positive electrode: Ni(OH)2+OH−→NiOOH+H2O+e−            negative electrode: M+H2O+e−→MH+OH−                        Discharging                    positive electrode: NiOOH+H2O+e−→Ni(OH)2+OH            negative electrode: MH+OH−M+H2O+e−                        
Much work has been completed over the past decade to improve the performance of nickel metal hydride batteries. Optimization of the batteries ultimately depends on controlling the rate, extent and efficiency of the charging and discharging reactions. Factors relevant to battery performance include the physical state, chemical composition, catalytic activity and other properties of the positive and negative electrode materials, the composition and concentration of the electrolyte, the separator, the operating conditions, and external environmental factors. Various factors related to the performance of the positive nickel hydroxide electrode have been considered, for example, in U.S. Pat. Nos. 5,348,822; 5,637,423; 5,905,003; 5,948,564; and 6,228,535 by the instant assignee, the disclosures of which are hereby incorporated by reference.
Work on suitable negative electrode materials has focused on intermetallic compounds as hydrogen storage alloys since the late 1950's when it was determined that the compound TiNi reversibly absorbed and desorbed hydrogen. Subsequent work has shown that intermetallic compounds having the general formulas AB, AB2, A2B and AB5, where A is a hydride forming element and B is a weak or non-hydride forming element, are able to reversibly absorb and desorb hydrogen. Consequently, most of the effort in developing negative electrodes has focused on hydrogen storage alloys having the AB, AB2, AB5 or A2B formula types.
Desirable properties of hydrogen storage alloys include: good hydrogen storage capabilities to achieve a high energy density and high battery capacity; thermodynamic properties suitable for the reversible absorption and desorption of hydrogen at room temperature; low hydrogen equilibrium pressure; high electrochemical activity; fast discharge kinetics for high rate performance; high oxidation resistance; weak tendency to self-discharge; and reproducible performance over many cycles. The chemical composition, physical state, electrode structure and battery configurations of hydrogen storage alloys as negative electrode materials in nickel metal hydride have been investigated and reported in the prior art. Some of this work is described in U.S. Pat. Nos. 4,716,088; 5,277,999; 5,536,591; 5,616,432; and 6,270,719 to the instant assignee, the disclosures of which are hereby incorporated by reference.
Efforts to date indicate that intermetallic compounds are capable of effectively functioning as negative electrode materials in rechargeable batteries, but that important properties are difficult to optimize simultaneously. Hydrogen storage alloys of the AB5 type, for example, generally have high initial activation, good charge stability and relatively long charge-discharge cycle life, but at the same time have low discharge capacity. Furthermore, attempts to increase the cycle life generally lead to reductions in the initial activation. Hydrogen storage alloys of the AB2 type, on the other hand, typically possess high discharge capacity, but frequently suffer from low initial activation and relatively short cycle life. Although recent achievements in atomic engineering are improving the situation, efforts to improve upon the initial activation customarily have come at the expense of cycle life. Other important properties include discharge rate, discharge current, and constancy of output over time. It has proven difficult in most applications to simultaneously optimize all desired battery attributes and as a result, compromises are normally made in which some properties are sacrificed at the expense of others.
One aspect of rechargeable batteries for HEV, EV, 42 V SLI and other applications that has received relatively little attention is low temperature operation. For HEV and EV products it is desirable to have batteries that perform well in harsh winter climates. Similarly, achievement of portable and stationary power sources based on rechargeable batteries that are capable of functioning outdoors in cold climates or in indoor cold environments is also desirable. A basic limitation of virtually every battery technology is a diminution of power and performance at low temperature. The deleterious effects of temperature are especially pronounced below freezing. Like other rechargeable batteries, nickel metal hydride batteries suffer significant degradation in power and performance upon a lowering of temperature. Improvements in the low temperature performance require consideration of the underlying components and principles of operation of nickel metal hydride batteries.
A need exists for improved rechargeable batteries having higher powers and discharge rates at low temperatures. With respect to nickel metal hydride batteries, the barrier to low temperature performance appears to reside primarily in the operating characteristics of the negative hydrogen storage alloy electrode. In U.S. patent application Ser. No. 10/405,008 ('008 application), the disclosure of which is hereby incorporated by reference herein, modifications of nickel metal hydride electrode materials designed to improve the performance under low temperature conditions were disclosed. The '008 application teaches techniques for controlling the porosity of the interface region of nickel metal hydride electrode materials to improve the accessibility of electrochemically active species to catalytic sites. The porosity modifications of the '008 application were motivated by a hypothesis that the ability of electrochemical reactants to reach catalytic sites and the ability of electrochemical products to depart from catalytic sites could be improved by increasing the porosity of the oxide or other matrix supporting the catalytic metal (e.g. nickel) particles utilized in the electrochemical charging and discharging reactions of nickel metal hydride batteries. By increasing the porosity, it was believed that the migration or mobility of electrochemical reactants and products to and away from the catalytic sites could be facilitated, thereby leading to improved performance. Such facilitation was expected to be especially important at conditions utilizing low operating temperatures since these conditions naturally impede the mobility of molecular and ionic species present in electrolyte solution. The materials of the '008 application were shown to exhibit much improved power and discharge rates at low temperatures (e.g. −30° C.). Pertinent details of the materials of the '008 application as they pertain to the instant invention are described hereinbelow.
In addition to high power and high discharge rates, it is desirable to develop a nickel metal hydride negative electrode material having the maximum possible cycle life while still maintaining good overall properties, including low temperature power capability. A need exists to understand the cycle life limitations of metal alloys and to overcome them through new alloy compositions and/or further engineering of alloy microstructure.